Technical Field
The present invention relates to a drive device for an insulated-gate semiconductor element and a power converter, and particularly to a drive device for an insulated-gate semiconductor element, which is capable of driving in parallel a plurality of parallel-connected insulated-gate semiconductor elements evenly with a constant current, and a power converter.
Background Art
A power converter uses IGBTs (Insulated-Gate Bipolar Transistors) for power control, MOS-FETs (Metal-Oxide-Semiconductor Field-Effect Transistors) or other insulated-gate semiconductor elements as the semiconductor elements for controlling the load. Especially in a power converter for a high power load, generally a plurality of insulated-gate semiconductor elements are connected in parallel and driven in parallel.
FIG. 4 is a diagram showing a schematic configuration example of a power converter in which drive devices for a plurality of insulated-gate semiconductor elements are disposed in parallel.
According to this configuration example, a power converter 1 has a plurality of IGBTs 2a to 2n and a plurality of drive devices 3a to 3n for driving the IGBTs 2a to 2n respectively. The plurality of IGBTs 2a to 2n are disposed in parallel by connecting the collectors thereof to one another and connecting the emitters of the same to one another, wherein the gates of these IGBTs are connected to the outputs of the drive devices 3a to 3n, respectively. The collectors of these IGBTs 2a to 2n that are connected to one another are connected to a high power load 4, and the emitters of the same that are connected to one another are connected to a ground line GND.
The plurality of drive devices 3a to 3n receive a common drive signal and drives the IGBTs 2a to 2n in parallel, respectively. This causes the plurality of IGBTs 2a to 2n to function as a single power switching device that drives the high power load 4.
Here, the drive devices 3a to 3n control the IGBTs 2a to 2n by applying a predetermined voltage to the gates of the IGBTs 2a to 2n. On the other hand, there has been proposed a technique for lowering the possibilities of loss and noise that occur in the IGBTs 2a to 2n when the IGBTs 2a to 2n are turned ON, by supplying a constant current to the gates of the IGBTs 2a to 2n to turn the IGBTs 2a to 2n ON (see Japanese Patent Application Publication No. 2008-103895, for example).
FIG. 5 is a diagram showing a schematic configuration of a drive device for an insulated-gate semiconductor element in a system where the gates of conventional IGBTs are supplied with a constant current. Note that the plurality of drive devices 3a to 3n of the power converter 1 shown in FIG. 4 are configured in the same way; thus, FIG. 5 shows the configuration of the drive device 3a as a representative example.
The drive device 3a has a constant current circuit 5 for generating a constant current, a discharge circuit 6 for connecting the gate of the IGBT 2a to the ground, and a switching circuit 7 for complementarily turning the constant current circuit 5 and the discharge circuit 6 ON and OFF in response to a drive signal.
In the drive device 3a with the foregoing configuration, when a drive signal for turning the IGBT 2a ON is input, the switching circuit 7 turns the IGBT 2a ON by supplying the gate of the IGBT 2a with a constant current generated by the constant current circuit 5. On the other hand, when a drive signal for turning the IGBT 2a OFF is input, the switching circuit 7 activates the discharge circuit 6 to cause the discharge circuit 6 to connect the gate of the IGBT 2a to the ground and discharge the electric charge accumulated in the gate, thereby turning the IGBT 2a OFF.
In the drive device 3a with the foregoing configuration, supplying a constant current to the gate of the IGBT 2a turns the IGBT 2a ON, achieving a constant speed for charging the electric charge accumulated in the gate of the IGBT 2a. Therefore, unlike the conventional, typical drive method for turning an IGBT ON/OFF by controlling the gate voltage of the IGBT, the charging speed of the gate of an IGBT is not changed by a change in ON-resistance that depends on the temperature of a semiconductor element (transistor) that drives the gate of the IGBT. Therefore, the IGBT 2a can be turned ON at constant time regardless of a change in the temperature, lowering the possibilities of loss and noise that occur in the IGBT when the IGBT is turned ON.
Unfortunately, supplying a constant current to each of the plurality of parallel-connected IGBTs 2a to 2n to turn the IGBTs 2a to 2n ON raises the possibility that the current flows intensively to an IGBT of low gate threshold voltage due to variations in the gate threshold voltages caused by the individual difference among the IGBTs 2a to 2n. Such concentration of current at the time of turning the IGBTs ON has a risk of thermally breaking the IGBTs.
There has conventionally been proposed a technique for measuring and storing the gate current values of the plurality of IGBTs 2a to 2n in advance and controlling the gate currents of the IGBTs 2a to 2n based on these gate current values to achieve a current balance (see Japanese Patent Application Publication No. H11-235015, for example).
There has also been proposed a technique for offsetting the drive control voltages and emitter voltages of the IGBTs 2a to 2n with an equal potential in response to the difference between a target gate threshold voltage and each of the gate threshold voltages of the IGBTs 2a to 2n (see Japanese Patent Application Publication No. 2008-178248, for example). This technique can match the timings for turning the IGBTs 2a to 2n ON, achieving a current balance among the plurality of IGBTs 2a to 2n. 
There has also been proposed a technique for detecting a low-temperature element and a high-temperature element and supplying the high-temperature element with a drive signal with larger delay to interfere with the flow of the current therein, while supplying the low-temperature element with a drive signal with a smaller delay to allow the current to flow easily therein (see Japanese Patent Application Publication No. 2009-159662, for example).
There has also been proposed a technique for detecting a low-temperature element and a high-temperature element, supplying the low-temperature element with a delayed drive signal, increasing the switching loss that arises when turning the element OFF to increase the amount of heat generated, and thereby equalizing the temperature and current between the low-temperature element and the high-temperature element (see Japanese Patent Application Publication No. 2009-135626, for example).
There has also been proposed a technique for comparing the temperatures between two parallel-connected power MOS-FETs and lowering the gate voltage of the power MOS-FET of a higher temperature, to equalize the temperatures of the power MOS-FETs (see Japanese Patent Application Publication No. 2002-142492, for example).